Powder metal technology is well known to the persons skilled in the art and generally comprises the formation of metal powders which are compacted and then subjected to an elevated temperature so as to produce a sintered product.
Moreover, U.S. Pat. No. 2,289,569 relates generally to powder metallurgy and more particularly to a low melting point alloy powder and to the usage of the low melting point alloy powders in the formation of sintered articles.
The modulus of elasticity and toughness of conventional powder metal articles is limited by density. Conventional powder metal processes are limited to approximately 7.2 g/cc by single press, single sinter techniques. At added cost double press, double sintering can be used to increase density.
Furthermore various processes have heretofore been designed in order to produce sintered articles having high densities. Such processes include a double press double sintering process for densities typically up to 7.5 g/cc as well as hot powder forging where virtually full densities of up to 7.8 g/cc may be obtained. However, such prior art processes are relatively expensive and time consuming. Recently developed methods include warm pressing of powders to up to 7.35 g/cc as disclosed in U.S. Pat. No. 5,154,881 (Rutz). However there are process disadvantages with the warm pressing such as maintaining tool clearances with heated systems. Also warm pressing does not allow very high densities up to and above 7.5 g/cc to be easily reached in commonly used alloy systems without double pressing and double sintering.
Yet another process is disclosed in U.S. Pat. No. 2,027,763 which relates to a process of making sintered hard metal and consists essentially of steps connected with the process in the production of hard metal. In particular, U.S. Pat. No. 2,027,763 relates to a process of making sintered hard metal which comprises producing a spray of dry, finely powdered mixture of fusible metals and a readily fusible auxiliary metal under high pressure producing a spray of adhesive agent customary for binding hard metals under high stress, and so directing the sprays that the spray of metallic powder and the spray of adhesive liquid will meet on their way to the molds, or within the latter, whereby the mold will become filled with a compact moist mass of metallic powder and finally completing the hard metallic particle thus formed by sintering.
U.S. Pat. No. 4,707,332 teaches a process for manufacturing structural parts from intermetallic phases capable of sintering by means of special additives which serve at the same time as sintering assists and increase the ductility of the finished structural product.
Moreover, U.S. Pat. No. 4,464,206 relates to a wrought powder metal process for pre-alloyed powder. In particular, U.S. Pat. No. 4,464,206 teaches a process comprising the steps of communinuting substantially non-compactable pre-alloyed metal powders so as to flatten the particles thereof heating the communinuted particles of metal powder at an elevated temperature, with the particles adhering and forming a mass during heating, crushing the mass of metal powder, compacting the crushed mass of metal powder, sintering the metal powder and hot working the metal powder into a wrought product.
Other methods to densify or increase the wear resistance of sintered iron based alloys are disclosed in U.S. Pat. No. 5,151,247 which relates to a method of densifying powder metallurgical parts while U.S. Pat. No. 4,885,133 relates to a process for producing wear-resistant sintered pans.
The processes as described in the prior art all have serious shortcomings in cost effectively producing the desired mechanical properties of the sintered product.
It is a further object of this invention to provide a process for producing sintered articles of densities greater than 7.4 g/cc by a single compaction, single sinter process. It is an object of this invention to provide an improved process for producing sintered articles having improved dynamic strength characteristics and an accurate method to control same.
It is a further object of this invention to provide an improved process for producing sintered articles having improved strength characteristics with carbon contents above 0.8% and in particular between 0.8% to 2.0% carbon and an accurate method to control same.
It is a further aspect of this invention to produce PM articles with high ductility by spheroidization.
Historically steels have been produced with carbon contents of less than 0.8%. However ultrahigh carbon steels have been produced. Ultrahigh carbon steels are carbon steels containing between 0.8% to 2.0% carbon. The processes to produce ultra high carbon steels with fine spheroidized carbides are disclosed in U.S. Pat. No. 3,951,697 as well as in the article by D. R. Lesver, C. K. Syn, A. Goldberg, J. Wadsworth and O. D. Sherby, entitled "The Case for Ultrahigh-Carbon Steels as Structural Materials" appearing in Journal of the Minerals, Metals and Materials Soc., Aug. 1993.
Applicant has filed PCT application No. PCT/CA94/00065 on Feb. 7, 1994 as well as U.S. application Ser. No. 08/193,578 on Feb. 8, 1994 for an invention entitled HI-DENSITY SINTERED ALLOY concerning the process of forming sintered articles of powder metal by blending combinations of finely ground ferro alloys with iron powders to produce sintered parts in a reducing atmosphere to produce sintered parts having a high density.
It is an object of this invention to provide another improvement in producing high density sintered parts by use of pre-alloyed powder as the base material and adding graphite thereto.
The broadest aspect of this invention relates to a process of forming a sintered article of powder metal comprising blending graphite and lubricant with a pre-alloyed iron based powder pressing said blended mixture to shape in a single compaction stage sintering said article, and then high temperature sintering said article in a reducing atmosphere to produce a sintered article having a density greater than 7.4 g/cc.
It is another aspect of this invention to provide a process of forming a sintered article of powder metal comprising blending graphite of approximately 0.8% to 2.0% by weight and lubricant with a pre-alloyed iron based powder containing about 0.5% to 3.0% molybdenum, pressing said blended mixture to shape in a single compaction stage, sintering said article, and then high temperature sintering said article in a reducing atmosphere to produce a sintered article having a higher density.
It is another aspect of this invention to provide a powder metal composition comprising a blend of pre-alloyed iron based powder and graphite so as to result in an as sintered mass having between: 0.5% to 3.0% molybdenum; 0.8% to 2.0% graphite; remainder being iron and unavoidable impurities.